1. Field of the Invention
This invention relates to an improved cyclic urea synthesis process, in which urea is synthesized from carbon dioxide and ammonia in a stoichiometrical excess thereto.
2. Description of the Prior Art
Recently, separation of unreacted carbon dioxide and ammonia from a urea synthesis effluent in a urea synthesis process has been effected at more and more higher temperatures, and a typical example of the separation process is a stripping process with carbon dioxide gas or ammonia gas under urea synthesis pressures. In such a process as described above, inert gases contained in carbon dioxide and ammonia as starting materials such as nitrogen, hydrogen, and methane as well as a small amount of air added as a corrosion inhibitor in a urea synthesis autoclave (hereinafter referred to simply as an inert gas including the air) are accumulated in the steps operated at high pressures such as urea synthesis step, stripping step, or the like. Therefore, the inert gas is required to be withdrawn out of such steps.
There are two types of methods of withdrawing the inert gas, a first method in which the inert gas is separated in the urea synthesis autoclave, and a second method in which the inert gas is separated when unreacted carbon dioxide and ammonia stripped are condensed. The second method has such a drawback compared with the first method that the amount of carbon dioxide and ammonia entrained in the inert gas is increased.
Carbon dioxide and ammonia contained in the inert gas thus separated needs to be recovered. As one process for the recovery thereof, according to urea synthesis process of Snam Progetti S.P.A., the inert gas is separated on condensing unreacted carbon dioxide and ammonia stripped from the urea synthesis effluent with ammonia gas, and the inert gas thus separated is introduced into a medium pressure absorption column along with unreacted carbon dioxide and ammonia separated in medium pressure distillation to recover the carbon dioxide and ammonia contained in the inert gas ["Hydrocarbon Processing" 49 (No. 4) 115-116 (1970)].
As another process for the recovery thereof, according to a process of Stamicarbon N.V., an inert gas at the top of a urea synthesis autoclave is washed with an ammonium carbamate solution resulting from a low pressure carbon dioxide and ammonia recovery stage under urea synthesis pressures to absorb the carbon dioxide and ammonia contained therein, and the inert gas withdrawn from the absorption step is depressurized and then discharged (U.S. Pat. No. 3,691,729). It is necessary for this process to take an explosion-proof remedy because the inert gas, from which carbon dioxide and ammonia have been removed, has a danger of explosion, and to operate carefully because the process is performed under urea synthesis pressures and in an apparatus connected directly to the urea synthesis autoclave.